The development of aircraft identification and locating systems has often been associated with navigational systems. Originally, and to some degree in today's airspaces, pilots interact with ground based air traffic controllers (hereafter referred to as "ATC") over radio, and relay their position with respect to known navigational aids and their altitude. This works well when there are relatively few aircraft in the airspace. Another consideration is that certain navigational systems and altimeter equipment have inherent navigational inaccuracies. Considering that pilots at certain times cannot precisely provide their position makes aircraft separation (which is a responsibility of air traffic controllers) more difficult and unreliable.
As aircraft navigational equipment has become more precise, pilots have correspondingly become capable of more accurately relaying their position to the ATC. This progression traditionally leads to closer spacings between aircraft. The progression from relatively inaccurate to relatively accurate navigational equipment which pilots have relied upon include, but are not limited to, the systems described in the remainder of this paragraph. Dead reckoning relies heavily upon the pilot being able to see the ground. Automatic direction finders (ADF) are located aboard aircraft which pilots use to determine the relative heading to a fixed non-directional beacon (NDB) (for example, the NDB is presently located at a relative geolocations of 45 degrees off the nose of the aircraft). Pilots use very high frequency omni-directional range (VOR) equipment to determine where the aircraft is located (the aircraft is located on the 090 radial of the VOR). LORAN and GPS systems both provide relative positions of the aircraft relative to the earth (often in a latitude and longitudinal format.) The LORAN and the GPS systems are often associated with a computer system which provides other navigational information, such as the required relative heading to travel to a desired location.
The GPS can provide an aircraft with accurate geolocation and time-of-day (TOD) information. Consequently, GPS is gaining acceptance in the aviation community as a valuable navigation tool. However, during flight aircraft don't generally know the position, speed, and heading of other aircraft. Instead, aircraft rely on computer-coordinated scheduling and the ATC system with its human operators and ground-based radar systems to control congested areas. As the amount of scheduled and un-scheduled air-traffic continues to grow, so does the potential for aircraft collision.
The use of transponders has increased the sophistication and accuracy by which the relative positions of two aircraft can be determined. A transponder is carried aboard each aircraft, and provides a return beam to an interrogatory radar beam produced by a ground based radar facility. The return beam is received by the ground based radar facility and displayed to the ATC (but typically is not displayed to the pilots.) The time of flight from when the interrogatory signal is transmitted from the ground based facility to the time that the return signal is received by the ground based facility provides distance information which can be computed in the ground based facility. The azimuth of the aircraft relative to the ground based facility can be determined by using known techniques. Similarly, altitude encoding transponders are capable of providing the relative altitude of the aircraft above the ground based station.
Transponders provide very accurate information of the aircraft's distance, azimuth, and possibly altitude to the ground based facility. However, transponders are an interrogatory type system (involving interrogation signals and return signals), and as such require relatively complex and expensive fixed radar equipment to be associated with each radar transmitter and receiver (which limits transponder interrogatory transmitters to ground based systems or very sophisticated aircraft.)
Another use of transponders is related to the generation of wingtip vortices by aircraft. Landing aircraft generate wingtip vortices which result in a wake of turbulence. Wingtip vortices can result in rough landings for aircraft following the aircraft producing the wingtip vortices, especially in cases where a relatively light aircraft is landing or taking off soon after a larger aircraft on the same runway. The FAA has established certain requirements for spacing between successive aircraft using the same, or closely aligned, runways.
Transponders are used to provide aircraft spacing information to the ATC relating to wingtip vortices and the to protect against excessive turbulence created thereby. Unfortunately, transponders and the radar associated therewith often do not provide information to the controllers relating to the precise time at which each aircraft takes off or lands, since radar coverage often does not extend to the ground. Under these conditions, the ATC does not know precisely when a plane takes off or land, so the spacing (time at which following aircraft are permitted to take off or land on the same runway) is uncertain.
Overhauling the current ATC system is a costly proposition, so it is desired that any new system rely largely upon existing technology. Besides, it is not clear that a new ATC system of largely the same form as the present system would help to significantly limit confusion. One simple and relatively inexpensive solution is to keep the current ATC system in place, but equip all aircraft with some form of warning system that can alert a pilots of closely positioned aircraft which are following a potentially conflicting trajectory.
U.S. Pat. No. 5,153,823 which issued Oct. 6, 1992 to Fraughton et al.; and U.S. Pat. No. 4,835,537 which issued May 30, 1989 to Manion illustrate systems which present navigational information from a first aircraft to another aircraft. If there is a probability of a collision, an electronic warning will be provided on board the second aircraft. The difficulty with these systems is that they rely upon past and present tracking information only. The tracking velocity used in predicting future collisions in these systems will not take probabilistic future tracking into consideration (there is no change in predicted future aircraft velocity and/or direction based upon changes of velocities of the aircraft.) Considering that collision avoidance is often based upon split second decisions by both the pilot and ATC, more reliable predictions about future aircraft tracking would be highly desirable. In addition, both of these prior art patents utilize only a single receiver.
It would be highly desirable to provide a system by which aircraft and ATC could more accurately determine future projections of paths of travel of other aircraft, relative to their aircraft, in their vicinity. It would be preferably that this system be a non-interrogatory type system. Non-interrogatory type systems are also preferred since the channels required for these transmissions are numerous. In congested airspace, congested channels are capable of providing erroneous or conflicting information.